linux-kernel - Re: [PATCH 1/6] kernfs: create vm_operations_struct without page

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Date: Fri, 7 Jun 2024 21:18:04 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Logan Gunthorpe <logang@...tatee.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Martin Oliveira <martin.oliveira@...eticom.com>,
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, linux-mm@...ck.org,
	Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Tejun Heo <tj@...nel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Marciniszyn <mike.marciniszyn@...el.com>,
	Michael Guralnik <michaelgur@...dia.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Valentine Sinitsyn <valesini@...dex-team.ru>,
	Lukas Wunner <lukas@...ner.de>
Subject: Re: [PATCH 1/6] kernfs: create vm_operations_struct without
 page_mkwrite()

On Fri, Jun 07, 2024 at 10:16:58AM -0600, Logan Gunthorpe wrote:
> 
> 
> On 2024-06-06 23:03, Christoph Hellwig wrote:
> > On Thu, Jun 06, 2024 at 10:54:06PM +0200, Greg Kroah-Hartman wrote:
> >> On Wed, Jun 05, 2024 at 01:29:29PM -0600, Martin Oliveira wrote:
> >>> The standard kernfs vm_ops installs a page_mkwrite() operator which
> >>> modifies the file update time on write.
> >>>
> >>> This not always required (or makes sense), such as in the P2PDMA, which
> >>> uses the sysfs file as an allocator from userspace.
> >>
> >> That's not a good idea, please don't do that.  sysfs binary files are
> >> "pass through", why would you want to use this as an allocator?
> > 
> > I think the real question is why sysfs binary files implement
> > page_mkwrite by default.  page_mkwrite is needed for file systems that
> > need to allocate space from a free space pool, which seems odd for
> > sysfs.
> 
> The default page_mkwrite in kernfs just calls file_update_time() but, as
> I understand it, the fault code should call file_update_time() if
> page_mkwrite isn't set. So perhaps the easiest thing is to simply not
> add a page_mkwrite unless the vm_ops adds one.
> 
> It's not the easiest thing to trace, but as best as I can tell there are
> no kernfs binary attributes that use page_mkwrite. So alternatively,
> perhaps we could just disallow page_mkwrite in kernfs entirely?

Sure, let's do that.